The basic differential equation used in transport theory is the radiative transfer equation (RTE), which is equivalent to Boltzmann’s equation used in the kinetic theory of gases and neutron transport [1]}, [2]}, [3]}.
The most fundamental quantity is the specific intensity \(I(\mathbf {r,\hat{\mathbf {s}}})\)  that describes the average power flux density at position \({\mathbf {r}}\)  in a given direction \(\mathbf {\hat{\mathbf {s}}}\)  within a unit solid angle and a unit frequency band [1]}.
Due to its dependency on both the position \(\mathbf {r}\)  and the direction \(\hat{\mathbf {s}}\) , it is challenging to solve \(I(\mathbf {r,\hat{\mathbf {s}}})\)  directly.
